Various sorts of positive and negative interactions between microorganisms and plants/animals have been described previously. Similar to humans, microorganisms communicate with one another and result in both beneficial and detrimental connections. In this context, the following interactions and interrelationships have been discussed:
1. Symbiosis between Alga and Fungus (Lichens)
- Lichen is a thallus comprising two organisms, a fungus and an alga, that combine to produce a self-sustaining organism.
- Mycobiont refers to the fungal component, while phycobiont refers to the algal companion.
- The two groups of organisms coexist and appear as though they are a single plant. The lichen’s thallus is formed by a fungus, whereas the alga comprises just 5-10% of its bulk.
- The mycelium of the fungus produces a network that resembles tissue. Embedded within this dense clump of mycelium are algal cells.
- In general, fungi acquire their sustenance either saprophytically from decomposing organic matter or parasitically from a living host. In lichens, however, the alga provides sustenance for the mycelium.
- The algae cells produce their own food and/or fix atmospheric nitrogen, which is then distributed into fungal hyphae.
- This style of nutrition is known as biotrophic nutrition and is observed in lichen. Cyanophyta or Chlorophyta comprises the lichen-forming algal species. However, unicellular or filamentous forms are possible.
- The blue-green algae genera include Nostoc, Gloeocapsa, Rivularia, and Stigonema. Trebouxia species are the most frequent unicellular green algae among green algae.
- The majority of the lichen-forming fungi are Ascomycetes, with a few taxa of Basidiomycetes also present.
- No Phycomycetes fungus is involved in lichen production. Algae offer nourishment for fungi, while fungi give refuge for algae.
Based on the nature of their fungal companion and the forms of their fructification, lichens are split into two groups:
- Ascolichens (with an Ascomycete fungus component)
- Basidiolichens (in which the fungal component is a Basidiomycete).
However, lichens are split into three classes based on habitat:
- Saxicolous (growing on rocks or stones).
- Corticolous (growing epiphytically on the leaves and bark of trees).
- Terricolous (growing on soil).
Similar to lesser plants, the plant body of lichens is known as the thallus. The thalli of lichens are grey or greyish green in colour. On the basis of thalli structure, there are three major types of lichens:
- Custose lichens (thalli without lobes, growing on stones, boulders, bark, or any other hard substratum, and resembling a crust, such as Haemmatomma puniceum and Graphic scripta).
- Foliose lichens, including Chaudhuria, Cetraria, Parmelia, Peltigera, Physcia, and Xanthoria (thalli are flat, lobed, and leaf-like, appearing as twisted leaves, with a distinct lower and top surface, linked to substrate with rhizoid-like structures called rhizinae).
- Fruticose lichens (thalli are most prominent, most complex, and slender and freely branched; branches are cylindrical, flattened, and create a thread-like tuft; thalli are not separated between upper and bottom surfaces; examples include Cladonia, Ramalina, and Usnea).
2. Antagonistic Interactions (Antagonism)
- The composition of a habitat’s microflora and microfauna is determined by the biological equilibrium formed by the interactions and relationships of all the organisms within a community.
- The environmental factors, however, disrupt the equilibrium. This activity is known as antagonism. Nature’s balance mechanism is antagonism.
- Through this technique, some type of biological equilibrium is preserved. Amensalism, rivalry, and parasitism and predation are the three aspects of antagonism.
a. Amensalism (Antibiosis and Lysis):
- Amensalism is the phenomena in which one microbial species is negatively impacted by another, while the second species is unaffected by the first.
- In most cases, amensalism is achieved through the release of inhibitory chemicals such as antibiotics, etc.
- Antibiosis is a condition in which the metabolites generated by organism A inhibit organism B, but have no effect on organism A. (Photoplate 28.2).
- Chemically hazardous metabolites enter the cell membrane and hinder its action. Antibiotics, siderophores, enzymes, etc., are typical antimicrobial metabolites produced by microbes.
- Trichoderma harzianum and T. viride are powerful antagonists that are known to release cell wall lysing enzymes, such as -1, 3-glucanase, chitinase, etc. Enzymes are responsible for the lysis of fungal mycelium.
- Siderophores are extracellular secondary metabolites produced by bacteria (e.g. Aerobacter aerogenes, Arthrobacter pascens, Pseudomonas cepacia, P.fluorescens), Actinomycetes (Streptomyces spp. ), yeast (Rhodotorula spp. ), fungus (Penicillium spp. ), and dinoflagellates (Prorocentrum minimum).
- Siderophores are frequently referred to as iron-chelating compounds in microorganisms due to their high affinity for Fe3+ ions and low affinity for Fe2+ ions. Siderophores are compounds with a low molecular weight. These carry iron (III) into bacterial cells following chelation. Kloepper (1980) was the first to demonstrate the significance of siderophore synthesis by PGPR to plant growth enhancement.
- Siderophores bind Fe2+ and render other microbes Fe3+ deficient. As a result, the growth of microbes is inhibited. When siderophore-producing PGPR is present on the surface of the root, it provides iron to the plant.
- Consequently, plant growth is promoted. Photoplate 28.2 displays, for instance, siderophore release by Pseudomonas fluorescens and growth inhibition of Macrophomina phaseolina (creating a clear zone). In recent years, the role of siderophores in the biological control of plant diseases has grown substantially.
There is rivalry among microbes for nutrients, such as oxygen and space, but not for water potential, temperature, or pH. The success of a species in competing for substrate is influenced by its competitive saprophytic ability and inoculum potential.
Garrett (1950) proposes four characteristics that may contribute to the competitive saprophytic ability:
- Rapid germination and development of immature hyphae toward a source of soluble nutrients.
- Appropriate enzyme equipment for breakdown of carbon components of plant tissues.
- Secretion of fungistatic, bacteriostatic, and antibiotic growth products.
- Tolerance of fungistatic chemicals produced by microorganisms in competition.
Consequently, there is rivalry for limited resources. The insufficiency of readily accessible carbon compounds is the most likely cause of competition. When both rapid and slow-growing plants are placed to soil that has been sterilised and contains a low level of carbon, the fast-growing plants typically keep the slow-growing plants in check.
However, there is no such check on the less active heterotrophs when there is sufficient carbon supply. Under these conditions, the relationship between competitiveness and growth rate is straightforward.
c. Parasitism and Predation
Parasitism is a phenomena in which one organism consumes another, typically in a mild and non-debilitating way. Predation is an apparent kind of antagonism in which a live organism is mechanically assaulted by another organism, resulting in the latter’s death.
It is frequently a destructive and violent relationship. These phenomena are illustrated using fungi, amoebas, and nematodes.
i. Mycoparasitism (Fungus-Fungus Interaction)
- Mycoparasitism refers to the occurrence that occurs when one fungus is parasitized by another.
- The parasitizing fungus is referred to as hyper parasite, whereas the parasitized fungus is referred to as hypoparasite. Mycoparasitism is prevalent in nature.
- Several events result from the contact between fungi, including coiling, penetration, branching, sporulation, resting body creation, barrier formation to prevent pathogen access, and lysis of host cell (s).
- In coiling event (A), the hyperparasite i.e. antagonist (a) recognises its host hypha i.e. hypoparasite (h) within the microbial community, makes contact with the host hypha, and coils around it. The molecular basis of host recognition by the antagonist has been addressed. Manocha (1985) has provided the basis for mycoparasite host recognition.
- The cell wall surface of both host and non-host microorganisms contains D-glucose and N-acetyl-D-galactosamine residues as lectins; an antagonist recognises the appropriate spots (lectin residues) and binds the host hypha.
- As a result of coiling, the strength of the host hypha is diminished. The antagonist dissolves the cell wall of the host and enters its lumen.
- Occasionally, the host builds a resistive barrier (Fig. 28.4C) to inhibit penetration and proliferation within the lumen.
- Depending on nourishment, the antagonist produces branches and sporulates (s) within the host hypha (D).
- Until the host’s nutrients are depleted, the antagonist develops resting bodies (survival structures), such as chlamydospores (c), within the host hypha (E) due to nutrient depletion and loss of vitality for survival.
Mycophagy is the phenomena of amoebae feeding on fungus. It is known that numerous amoebae feed on harmful fungus. Arachnula, Archelle, Gephyramoeba, Geococcus, Saccamoeba, Vampyrella, etc. are hostile soil amoebae.
These amoebae interact with hyphae of the fungus and create holes. On Cochliobolus sativus, Gaeumannomyces graminis var. tritici, Fusarium oxysporum, and Phytophthora cinnamomi perforations have been seen. On this fungi’s lysed hyphae, amoebae create spherical cysts.
Chakraborty (1983) outlined the three key processes soil amoebae take when feeding on fungal propagules:
- Attachment: By accident, amoeba trophozoites adhere to fungal propagules, such as conidia, hyphae, etc. Attachment takes place via chemotaxis or thigmotaxis.
- Engulfment: According to their size, fungal propagules are completely ingested by amoebae. Small trophozoites connected to the hyphal wall or spore perforate it, however.
- Digestion: Inside the cysts, a huge central vacuole digests the wholly or partially ingested propagules and cytoplasm of the host fungal.
- The phenomena of fungi devouring nematodes is known as nematophagy, and the fungi are known as predatory fungi. Fungi are mechanically involved in attacking and destroying nematodes, which leads to nematode consumption.
- The predatory fungi are abundant in surface litter and decomposing organic materials. There are around fifty types of fungi known to attack nematodes. Different phases of nematode development are susceptible to assault by distinct species of fungi.
- M.S. Woronin demonstrated for the first time in 1869 that predaceous fungi capture and destroy worms using specialised organs.
- During the 1930s, C. Drechsler significantly expanded the list of predatory fungi and uncovered their trapping mechanism. Duddington (1957) reviewed the work of nematophagous fungi and added greatly to our understanding of these organisms.